1. Field
The present invention relates generally to the field of telecommunications, and more particularly to mechanisms for demodulating and decoding data transmitted over a link in a wireless communication system, where the link has multiple channels with varying characteristics.
2. Background
Wireless communication technologies are rapidly advancing, and wireless communication systems are utilized to provide a larger and larger portion of the communications capacity that is currently available to users. This is true despite the additional technological impediments that are faced in implementing a wireless communication system, as compared to a wireline system. For instance, personal telephone communications increasingly employ cellular telephones. Further, with advancements in cellular telephone, or more accurately, wireless communication systems, additional capabilities such as mobile internet access or high-speed data communications are becoming available to users.
One type of wireless communication system comprises a cellular cdma2000 (code division multiple access) system which is configured to support voice and data communications. This system may have multiple base stations which communicate via wireless channels with multiple mobile stations. Each base station/mobile station pair is communicates via a wireless link that may include multiple channels. These channels may be dedicated to various different types of data, such as voice data, high-speed data, control data, and so on.
Typically, a wireless link utilizes both a data traffic channel and a pilot channel to enable communications between a mobile station and a base station. The data traffic channel (or simply “traffic channel”) carries a signal which embodies the data that needs to be communicated between the mobile station and the base station. The pilot channel is a known signal that is used as an amplitude and phase reference for demodulating the data on the traffic channel. The pilot may therefore be referred to herein as a demodulation reference.
The power required to receive a data channel with a given quality is a function of the data rate, the signal to noise ratio (SNR) of the data channel, and the quality of the demodulation reference. For a given data rate, a larger SNR on the data channel will improve the quality of reception of the data. Similarly, for a given data rate, a larger SNR on the pilot channel will improve the accuracy of the demodulation reference, and thereby improve the quality of reception of the data.
The above is true for data being transmitted at a given data rate. It should be noted that, as the data rate increases, it is necessary to improve the quality of the modulation reference just to maintain the quality of the data demodulation. Thus, the SNR of the demodulation reference can be lower for lower data rates, but must be increased at higher data rates. For example, if a system increases its data rate from 9600 to 76,800 bps, the power of the pilot signal must be increased by 9 dB (thereby increasing its SNR by 9 dB) in order to maintain the quality of the data demodulation.
While the quality of the demodulation of the data signal can be improved by increasing the power of the demodulation reference, this should be balanced with a desire to conserve power and to minimize interference with other communications. The capacity of wireless systems is typically maximized by minimizing the amount of energy required to receive one bit of data with the desired quality. A low energy per bit allows a transmitter with a limited power to send bits at a faster rate, thereby achieving a higher data rate. In addition, a lower energy per bit will allow the transmitter to radiate less energy per bit, thereby reducing the power consumption and the interference to other devices.
The need for a demodulation reference to extract the data from a received signal raises additional issues. One such issue relates to the use of multiple traffic channels. If it is desired to have multiple traffic channels, it is likely that a single demodulation reference cannot maintain a power level that will always be optimal for each of these channels. In particular, this is true if one of the traffic channels carries relatively low-speed data, while another carries high-speed data. Demodulation of the low-speed data requires a lower-power demodulation reference, while demodulation of the high-speed data requires a higher-power demodulation reference.